1. Field of the Invention
The present invention is related to crystal oscillators, and more particularly, to digitally controlled temperature compensated oscillators.
2. Background Art
Many commercial oscillators are based on LC type oscillators. The “C” in the LC oscillator, or the capacitance, is usually implemented as a varactor, which is essentially a reverse-biased diode. The capacitance of the varactor is voltage controlled, by controlling the reverse biasing of the diode.
The voltage for controlling the varactor is usually generated by a temperature controlled sensor. The “L” (inductance) is usually due to the size of the crystal itself, although sometimes a separate inductive element is used. In other words, the LC oscillator is compensated by the temperature.
However, one difficulty with commercially available LC oscillators is that it is difficult to match the temperature dependence of the inductance (L) to that of the C. Typically, the temperature dependencies of the L and C are of an opposite sign, and the circuit designer tries to match the two dependencies such that the overall response of the LC oscillator is flat over some specified temperature range.
In practice however, such a precise matching is very difficult. The presence of the mismatch in the temperature dependence of the L and the C results in a temperature dependence of the frequency of the output, which in turn results in distortions and nonlinearities in circuits that rely on a precise clock waveform. Accordingly, a need exists for an oscillator whose frequency response is flat over the specified temperature range.